1. Field of the Invention
The present invention relates to a cascade source and a method for controlling the cascade source.
2. Description of the Related Art
A cascade source is known from practice. The original cascade source was invented by Maecker in 1956. Subsequently, an argon plasma source was developed from this by Kroesen et al. The known cascade source is provided with a copper cathode housing and three cathodes provided with 0 tungsten tips reaching into the cathode housing. In the known apparatus, the cascade plates are manufactured from copper and contain cooling channels through which water can be led for cooling cascade plates.
Between each two copper plates stacked on top of each other, an O-ring, an insulation plate of, for instance, PVC and a boron nitride plate are present which together provide vacuum sealing and electrical insulation. The plasma arc extends between the tips of the cathodes and the outflow opening of the anode. In general, the cascade source is connected to a process chamber in which a strongly reduced pressure prevails. Into the cathode housing, a fluid is supplied under higher pressure. This fluid flows from the cathode housing via the plasma channel to the process chamber at a high speed. As a result of this gas flow, the plasma extends far into the process chamber, so that it is active there.
In the known cascade source, the three cathodes are all insulated with respect to the copper cathode housing. Because the distance between the conductive cathode housing and the electrode tips of the cathodes is very small, in the known source, there is a considerable chance that, during the ignition of the plasma, for a short time, disruptive discharge takes place between the electrode tip and the cathode housing. Such a disruptive discharge is accompanied by sputtering of the electrode tip, which considerably shortens the life of the electrode tip. In addition, as a result of the sputtering, copper or electrode material can end up in the processing environment, which can have disastrous consequences for the substrate to be treated in the process chamber. Thus, in the known source, the cathodes had to be replaced regularly. The replacement of the cathodes and the subsequent repositioning of the electrode tip in the cathode housing is, in the known apparatus, a time-consuming and difficult job. This is inter alia caused by the fact that, when demounting the cathode housing, the mutual connection between the cascade plates was also lost.